1. Technical Field
The present invention relates to tracking the precise location of assets by either tracking the equipment that moves them or tracking the assets themselves. More particularly, the present invention relates to the tracking and inventorying of containers, within container storage facilities, by combining multiple positioning techniques that involve using a satellite Global Positioning System (GPS), an Inertial Navigation System (INS), or other movement or status sensors for position determination.
2. Related Art
Position or location tracking is a crucial component of inventory or resource management systems. Location tracking systems can employ real-time positioning sensors that continuously or periodically provide position data for tracking the location of resources. These systems commonly acquire the locations of vehicles, equipment, or inventory based on principles of either triangulation or proximity with respect to known locations determined from sensors. Typical sensors used include Global Positioning System (GPS), Differential Global Positioning System (DGPS), Integrated Differential Global Positioning System and Inertial Navigation System (DGPS/INS), Real Time Locating System (RTLS), RTLS/GPS, RTLS/INS, transponders and ultra wideband locating systems. As one example, U.S. Pat. No. 6,577,921 discloses a container tracking system that tracks the real-time positions of the container handling equipment using both GPS and INS sensors.
Limitations in physics, however, generally prevent real-time positioning systems from achieving 100% reliability or accuracy. Examples of those limitations with respect to radio-wave positioning include obstacles blocking line of sight position signals, or signals reflected from nearby surfaces (multi-path). Further practical limitations in sensor technologies include electromagnetic and other inherited noise and biases in measurements, as well as a poor signal to noise ratio resulting from environmental interference sources. These limitations result in common positioning errors such as inaccuracies, loss of position, or location drifts causing erroneous position or inventory data.
Even when more accurate real-time position data can be determined through a combination of sensors, data and methods, these solutions do not solve another important problem in the inventory and resource tracking environment, namely what happens when at least a portion of the real-time position sensor data is inaccurate, missing or is lost? As a simple example, a real-time positioning system based on an expensive integrated GPS/INS solution can drift away from the true positions when the system enters an area covered by less than four GPS satellites for a long period of time. In a typical inventory tracking environment, inaccurate location measurements, if not corrected in time, can propagate into widespread inventory location errors. This occurs especially when tracking the position of containers or vehicles in a warehouse, container yard, or rail yard where tracking signals are often blocked. Resultant errors then require manual correction. The correction of the corrupted inventory database can create delays and often expensive corrective measures in resource management and inventory controls.
It is desirable to provide a system that increases the accuracy of monitoring the real-time position of an object such as a cargo container. It is further desirable to improve the accuracy of inventory location monitoring when one or more sensors, such as a GPS sensor, is unavailable or highly inaccurate due to noise.